Philosophers are flummoxed by hallucinations. You should be too. To see why, create one. Hold up a single finger before your eyes and, if you focus on an object behind it, it will appear as two ghostly fingers. Inspect the ghost fingers. Can you tell which is the real one just by looking? Don't cheat by blinking, just look. You should find that they are perfectly indistinguishable.

This should worry you because, as you know, they are also perfectly different. To see this difference (or better feel it) take a "touch test"; choose which finger you take to be real and, using a finger on your other hand, try to touch it. You'll either touch nothing or your finger. That's because they are completely different kinds of things, even though they are completely similar looking things. (In an attempt to make this review accessible to non-philosophers, I’ve left off motivating disjunctivism in the traditional way using Cartesian skeptical cases, like a deceptive being removing the object of perception while preserving all of its effects in the brain.) Baffled? Good. Now you are at least feeling like a philosopher. To be one, you'll need to resolve this problem, or at least try.

The collection of papers represented in this volume centers on a recent philosophical account of hallucinations called "disjunctivism", the claim that there is no common element that constitutes the perceptory, object-based experience and the non-object-based hallucinatory experience. The disjunctive view, as one of the authors A.D Smith argues, is an account that preserves our judgments about the disparity between these indistinguishable but distinctive experiences:

"[T]he disjunctive theory proposes that we best express what our subject definitely does know in such a situation as his knowing either that he is perceiving something of such and such a sort or that he is hallucinating such a thing." (168)

Disjunctivism represents a clean break (or as clean a break as philosophers make--there are moderate positions, like the one Mark Johnston proposes in his contribution) from the traditional "Cartesian" or "sense data" account--defended in this volume by Alan Millar and Howard Robinson--where there is a common element, the sense data. Both ghostly flesh-colored finger shapes, on this view, are the same kind of object causing the same type of experience. In doing so, however, sense data act as veils to direct experiences of external objects. Disjunctivism, on the other hand, opens up the conceptual resources for a common sense or naive realist claim that objects are directly available, which can handle hallucinations, or other similar problems, a mark in its favor as several of the authors, John McDowell, J.M. Hinton, Paul Snowdon and M.G.F. Martin can be taken to argue.

What is odd about disjunctivism, however, is its claim that the perceptory and hallucinatory experiences are judged to be distinct but are experienced as indistinguishable. If we remove this reflective judgment component, as Mark Johnston suggests we do, we seem to be left with just the same experiences, offering no account of hallucinations at all. A "mind-blind" animal, for instance, lacking any judgment concerning its sensory experiences, cannot be taken to be having a hallucination when it misperceives on this account. And yet, argues Johnston, "hallucination is a distinctive kind of mental act" (219) requiring an account that can show what it is about the hallucination itself that is unique.

Disjunctivism has only the etiology of hallucinations--the mere act of having a hallucination--to account for their nature. In response, some authors in this volume, like Jonathan Dancy and Harold Langsam, have augmented disjunctivism with other philosophical accounts of perception into a positive account of hallucinations--an account that takes hallucinations to be a unique kind of object. Supplementing disjunctivism with Alston's "Theory of Appearing," where perception is a particular relation between a perceiver and an appearance of an object, Langsam offers a new understanding of a hallucinatory experience as, "a certain relation coming to obtain between a region of physical space and myself." (193) In this sense, the hallucination's relation to me is something real, but different than a percept's relation between an object and myself in that they share distinct objects. This positive account of disjunctivism has then something more on offer for its detractors and, I will argue, for empirical research.

"Positive disjunctivism", as the editors Alex Byrne and Heather Logue have termed it in their helpful introduction, can, for instance, account for the doubled finger hallucination as a relation between oneself and, say, an optically-derived image. This account, I think, grounds further investigation into the nature of this image's mental basis; if there is something that exists as a relation between me and a hallucinated object, then there is something to be manipulated empirically--something to produce results.

How might hallucinations be manipulated? As one suggestion, try creating an afterimage of one. In a darkened room, put yourself in a position to see your two ghostly fingers and flip the light on for a moment, then flip it off. Your two-finger afterimage should be floating around in the darkness. The previously indistinguishable ghostly fingers should have changed slightly: one should have a more substantial look and the other a less substantial look. In other words, you can now distinguish between the two experiences, the substantial looking finger is an afterimage of the real finger as can be discovered by remembering which side it is on and using the touch test.

Positive disjunctivism, then, suggests a basis for empirically verifying its claims. The disjunctivist view can account for these results by claiming that whatever brain processes involved in making the hallucinated and perceived fingers indistinguishable was inactive during the production of the two-finger afterimage, showing them to be, in fact, two different kinds of experience. Discovering what specific brain processes were inactive might further verify positive disjuncitivism.

Though this review focused on hallucinations, both illusions and delusions are central points of analysis in this outstanding collection of papers in the philosophy of perception. I strongly recommend this volume to all those studying perceptual anomalies.

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